Thermography

Thermography turns heat into a picture. Where a thermocouple or a pyrometer reports the temperature of one point, a thermographic system reports the temperature of every point in its field of view at once, fast enough to follow a moving weld pool or a cooling bead. That property is what makes it useful for welding: a defect, a cold spot, an uneven heat-affected zone or an out-of-band interpass temperature usually shows up as a shape in the thermal field before it shows up as anything else.

The technique splits into two families. Passive thermography reads the heat the process itself already produces — the arc, the weld pool, the cooling bead — and needs no external stimulus. Active thermography injects heat or another excitation into a finished part and watches how it diffuses, which is what makes subsurface and post-weld inspection possible on a joint that has already cooled to ambient. The two answer different questions, and neither replaces the other.

What a thermographic system actually measures is infrared radiance, not temperature. Converting radiance into temperature requires knowing the surface’s emissivity, which changes with material, oxidation state and viewing angle — the reason a serious thermal weld-monitoring setup is built to be robust to that uncertainty rather than to assume a fixed value. Which instrument captures the radiance also matters: an infrared camera produces a full field, while a single-point sensor produces one reading, and the thermal gradient across a joint is only visible to the former.

Thermography sits alongside, not instead of, the other NDT families — visual, radiographic, ultrasonic and magnetic methods each answer questions thermography cannot, and a full inspection plan combines routes rather than substituting one for another. The dedicated thermography and NDT testing page compares those methods and where each is chosen for weld inspection.